Message: "Norton Extension could not be installed because it is not compatible with Firefox 57 or Nightly Firefox 58 version"

Older versions of Norton extension for Firefox do not work after you upgrade it to Firefox 57 on Mac OS 10.9.x. To fix this problem, manually remove the older version of Norton extension, and install the latest version.

For Norton Internet Security 5.10.x and Norton Security 6.x, only Norton legacy extensions are available that are not compatible with Firefox 57 or later.